In "For Whom the Bell Tolls," a main quest in Kingdom Come: Deliverance 2, Hans requires your assistance at Trosky Castle. A crucial step involves creating a Fever Tonic. Here's how:

Locating the Fever Tonic Recipe:

The recipe resides within the Surgeon's Workshop in Trosky Castle. Access can be gained through stealth or by persuading the chamberlain. After delivering the horseshoe in the "For Whom the Bell Tolls" quest, speak with Cook Fanka. Assisting her with a locked chest grants access to the noble's kitchen, where the ailing chamberlain can be found. Successfully completing skill checks will convince him to allow entry to the workshop. Alternatively, if skill checks fail, steal food, proceed to another kitchen area (accessed via a different set of stairs), bribe the guard with the stolen food, and sneak into the workshop. The Surgeon's journal, containing the recipe, is located on a bookshelf.

Brewing the Fever Tonic:

The necessary ingredients are already present on the surgeon's station. Follow these steps:

1. Pour wine into the cauldron.
2. Add 3 handfuls of Feverfew.
3. Lower the heat and boil for 2 turns of the sandglass.
4. Grind a handful of Elderberry Leaves using the mortar and pestle.
5. Add the ground Elderberry Leaves.
6. Add 2 handfuls of Ginger.
7. Obtain a phial from the bottom-left shelf.
8. Distill the tonic using the Still. Additional herbs are available in a chest if needed.

Completing the Quest:

Having brewed the Fever Tonic, locate Thomas. If the chamberlain was persuaded, return with both the Fever Tonic and a Digestive Potion. Otherwise, use the noble's kitchen to access the chapel and find Thomas and his sister, Adele. Speak with Adele to conclude the quest.